1. Field of the Invention
The present invention relates to a polymeric fluorescent substance, a polymer light emitting device (hereinafter, referred to as polymer LED) containing the same in an light emitting layer, and a device using the same.
2. Description of the Related Art
Inorganic electroluminescence devices (hereinafter, sometimes referred to as inorganic EL device) using an inorganic luminescent material as a light emitting material are used for example for flat light sources as back-lights and displays such as a flat panel display and the like, however, an alternating current of high voltage has been required for light emission.
Recently, Tang et al. fabricated an organic electroluminescence device (hereinafter, sometimes referred to as organic EL device) having a double-layer structure in which an organic fluorescent dye as a light emitting layer is laminated with an organic charge transport compound used in photosensitive layer for electrophotography and the like (Japanese Patent Application Laid-Open (JP-A) No. 59-194393). Since organic EL devices have characteristics that light emissions of a lot of colors are obtained easily in addition to low voltage driving and high luminance as compared with inorganic EL devices, there have been reported a lot of trials regarding device structures, organic fluorescent dyes and organic charge transport compounds of organic EL devices [Jpn. J. Appl. Phys., 27, L269 (1988), J. Appl. Phys., 65, 3610 (1989)].
Further, apart from organic EL devices using mainly organic compounds having a lower molecular weight, polymer light emitting devices (hereinafter, sometimes referred to as polymer LEDs) using light emitting materials having a higher molecular weight have been proposed in such as WO 9013148 published specification, JP-A No. 3-244630, Appl. Phys. Lett., 58, 1982 (1991) WO 9013148 discloses in the Examples an EL device using a thin film of poly(p-phenylene vinylene) obtained by forming a film of a soluble precursor on the electrode and subjecting it to a heat treatment to convert the precursor into a conjugated polymer.
Further, JP-A 3-244630 has exemplified conjugated polymer having a feature that they are themselves soluble in a solvent and need no heat treatment. Also in Appl. Phys. Lett., 58, 1982 (1991), a polymeric light emitting materials soluble in a solvent and a polymer LED fabricated using the same are described.
Polymer LEDs are advantageous for formation of a film having large area and reduction in cost since an organic layer can be easily formed by coating, as compared with the case of vapor deposition of a material having a lower molecular weight, and the mechanical strength of the resulting film is believed to be high because of a high molecular weight thereof.
Conventionally, as the light emitting materials used in these polymer LEDs, there have been reported polyfluorene (Jpn. J. Appl. Phys., 30, L1941 (1991)), poly p-phenylene derivative (Adv. Mater., 4, 36 (1992)) and the like, in addition to the above-described poly(p-phenylene vinylene).
Japanese Patent Application Laid-Open (JP-A) No. 5-202355 describes a polymeric fluorescent substance comprising a copolymer composed of 2 or more arylenevinylene repeating units, and as the arylene group, a biphenylene group and terphenylene group are exemplified.
JP-A No. 11-97175 describes a polyarylenevinylene composed of an arylene group or aromatic heterocyclic group substituted with two or more aryl groups or aromatic heterocyclic groups, and as the arylene group, a biphenylene group and terphenylene group are also exemplified.
JP-A No. 11-140168 describes a poly(substituted biphenylenevinylene) and a production method thereof.
JP-A No. 11-502248 describes a polymer containing an oligo-p-phenylene unit and a production method thereof.
JP-A No. 10-36487 describes an alternating copolymer of fluorenevinylene and arylenevinylene.
Though various polyarylenevinylenes and copolymers thereof have thus been disclosed as a polymeric fluorescent substance used on polymer LED, polymeric fluorescent substances which emits a further strong fluorescence have been required.
The object of the present invention is to provide a polymeric fluorescent substance emitting strong fluorescence, a polymer LED and a device having high performance which can be driven at low voltage at high efficiency by using the polymeric fluorescent substance.